It is often useful to use a number of different scan engines when an issue like this remains unresolved. If you search google for online scans (
like this you'll find zillions. Personally, I like
www.ewido.net/en/onlinescan because it searches for and removes Trojans, Worms, Dialers, Hijackers, Spyware and Keyloggers. It sounds likely to me that you've got a worm on your system (worms send mass emails from your system, often through their own servers).